Conventionally, mercapto compounds have been widely used as raw materials for synthesizing various medicines and pesticides. Especially, the usefulness of heterocyclic mercapto compounds as medicines is highly appreciated (Patent Document 2), but there are industrial problems with respect to the yields and the raw materials in the production thereof.
More specifically, the heterocyclic mercapto compounds are produced by reacting heterocyclic compounds with alkali salts of thiocarboxylic acids or with thiourea. Prior to the reaction, leaving substituents (for example, halogen group, mesyl group, tosyl group) other than target substituents of substitution reaction are protected and, after the reaction, the protected substituents are deprotected (Non-patent Document 1, Patent Documents 1 to 3). However, the production method using thiourea gives low yields of the objective compounds and is not industrially practical (Patent Document 1). Further, the alkali salts of thiocarboxylic acids are expensive, and therefore the production of the heterocyclic mercapto compounds therewith is unsatisfactory with respect to the industrial practicability (Non-patent Document 1, Patent Documents 1 to 3). In view of industrial practicability and costs, it is desirable that expensive sulfurizing agents represented by the alkali salts of thiocarboxylic acids be replaced by inexpensive sulfurizing agents such as metal sulfides and metal hydrosulfides. In the background art, however, use of sodium hydrosulfide as the sulfurizing agent has resulted in a low yield (Non-patent Document 1).    [Patent Document 1] U.S. Pat. No. 3,328,415    [Patent Document 2] JP-A-2002-543069    [Patent Document 3] JP-A-H04-103584    [Nonpatent Document 1] GEORG FUCH, “ARKIV FOR KEMI”, 26 (1966) (pages 111 to 116)